The below-mentioned Patent Document 1 discloses a structure in which, in a side airbag device, a balancing inflation chamber is formed at a lower rearward portion of a lower inflation chamber that opposes the pelvis area of a vehicle occupant. Thus, even if a flow of gas jetted from an inflator into the lower inflation chamber is diverted upward at a forward distal end portion of a dividing wall and a force pressing upward acts on the side airbag, the balancing inflation chamber applies a force pressing downward, and an upward movement of the lower inflation chamber is suppressed. Therefore, the airbag may be inflated and expanded in a state in which the lower inflation chamber is kept stable.    Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2007-210371